What's Wrong with Quaternion Rotation in the Game Engine?
From Hawkenfox:

"This issue seems to be an unsupported feature at the moment. Can someone mark this question as answered?" 

(Okay - I am posting this answer, pasted from your comment, for the purpose of marking this as an answer and closing the question. Please mark as answered.)

Brita wrote: "@gandalf3 I took a quick look, BGE indeed only supports the default euler rotations. The animation curves in BGE are Moguri's area, he is already assigned to that last bug.. so" – brita_ Oct 2 '14 at 8:00 – hawkenfox Dec 26 '15 at 3:10 

BTW, was this resolved?